I read tons of comments like "It's not [this], it's [that] instead!" which is also wrong. The performance bottleneck is whatever resource hits saturation first

I read tons of comments like "It's not [this], it's [that] instead!" which is also wrong. The performance bottleneck is whatever resource hits saturation first under the workload you actually run: CPU, memory bandwidth, cache/allocations, disk I/O, network, locks/coordination, or downstream latency. Measure it, prove it with a profile/trace, change one thing, measure again.